1. Field
The present application relates to systems and methods for panoramic imaging.
2. Background Art
Wide-angle and panoramic imaging have had significant impact on a variety of real-world applications. For example, being able to “see” in all directions provides situational awareness in surveillance and autonomous navigation tasks. Image-based immersive virtual reality enables realistic exploration of indoor and outdoor environments. Panoramic video conference and telepresence devices aid in effective virtual collaboration. All these applications rely on capturing high-quality panoramic videos. In many of these applications, it is not required to capture the entire spherical field of view (FOV). It is sufficient to capture 360 degree panoramas with a reasonable vertical field of view that has more or less equal coverage above and beneath the horizon. For instance, in the case of video conferencing, it is only necessary to capture the sitting participants and the table. The same is true for ground navigation, where the vehicle only needs to view the roads and the obstacles around it.
Current techniques for capturing panoramic videos can be classified as either dioptric (systems that use only refractive optics) or catadioptric (systems that use both reflective and refractive optics). Dioptric systems include camera-clusters and wide-angle or fisheye lens based systems. Catadioptric systems include ones that use single or multiple cameras with one or more reflective surfaces.
A popular dioptric solution is the use of fisheye lenses. Since fisheye lenses have a hemispherical FOV, fisheye lenses can only be used to capture hemispherical panoramas. D. Slater, “Panoramic Photography with Fisheye Lenses”, International Association of Panoramic Photographers, 1996, proposed the use of two back-to-back fisheye lens cameras to obtain full spherical panoramas. Similar to camera-cluster approaches, this approach suffers from large parallax. In addition, if the goal is to capture a reasonable vertical coverage about the horizon then in terms of optics, image detection and bandwidth, it is wasteful to capture the entire sphere.
Catadioptric system employ one or more reflective surfaces capable of projecting a wide view of the world onto a single sensor. Several of these designs satisfy the so-called single viewpoint (SVP) constraint (e.g., that the entire image appears to be capture from a single point of reference). S. Baker and S. Nayar, “A Theory of Single-Viewpoint Catadioptric Image Formation”, IJCV, 35(2):175-196, 1999, derived a family of mirrors that satisfy the SVP constraint, when used with perspective lenses. Since in these designs perspective cameras view the world through highly curved reflective surfaces, the vertical resolution (number of pixels per unit elevation angle) of the computed panoramas is not uniform and is, in general, poor near the apex of the mirror.
Yagi and Kawato, “Panorama Scene Analysis with Conic Projection,” IEEE International Workshop on Intelligent Robots and Systems, Vol. 1, pp. 181-187, 1990, and Lin and Bajsey, “True Single View Point Cone Mirror Omni-Directional Catadioptric System, ICCV, pp. II: 102-107, 2001, have used conical mirrors in their catadioptric designs, with the latter satisfying the SVP constraint. In these examples, a better vertical resolution is obtained. The conical mirror acts as a planar mirror in the radial direction and hence the vertical resolution is the same as that of the camera. However, this restricts the maximum vertical FOV of the panorama to half the FOV of the camera. For instance, if the goal was to capture a panorama with a 60 degree vertical field of view, the camera must have a 120 degree field of view. Even if such a system is used, the resolution of the panorama approaches zero at the apex of the cone. More importantly, conical mirrors are known to produce strong optical aberrations due to astigmatism that fundamentally limit the resolution of the captured panorama. Thus, there is a need for improved systems and methods for capturing panoramic images.